A. Technical Field
The present invention relates to a production process for a water- absorbing agent which is favorably used for sanitary materials such as paper-made diapers (disposable diapers), sanitary napkins and so-called incontinent pads.
B. Background Art
In recent years, water-absorbent resins are widely used as constituent materials of sanitary materials, such as disposable diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent resins to absorb body fluids.
Known examples of the above water-absorbent resins are as follows: crosslinked polymers of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile- or acrylamide copolymers, and their crosslinked polymers; and crosslinked polymers of cationic monomers.
Examples of the properties which the above water-absorbent resins should have are as follows: upon contact with aqueous liquids such as body fluids, excellent water absorption amount or speed, the liquid permeability, the gel strength of the swollen gel, the suction power to suck up water from a base material containing aqueous liquids. However, relations between these properties do not necessarily display positive correlations. For example, as the absorption capacity under no load increases, the absorption properties under a load deteriorate.
As to a method for improving such water-absorption properties of the water-absorbent resin in good balance, there is a known art in which the neighborhood of the surface of the water-absorbent resin is crosslinked, and various methods have been proposed as such.
For example, there are known methods in which the following materials are used as the crosslinking agents: polyhydric alcohols (JP-A-58-180233 and JP-A-61-016903); polyglycidyl compounds, polyaziridine compounds, polyamine compounds, or polyisocyanate compounds (JP-A-59-189103); glyoxal (JP-A-52-117393); polyvalent metals (JP-A-51-136588, JP-A-61-257235 and JP-A-62-007745); silane coupling agents (JP-A-61-211305, JP-A-61-252212, and JP-A-61-264006); alkylene carbonates (DE 4020780). In addition, there are also known methods in which the following materials are allowed to be present for the purpose of improving the dispersibility of the crosslinking agent during the crosslinking reaction: inert inorganic powders (P-A-60-163956 and JP-A-60-255814); dihydric alcohols (JP-A-01-292004); water along with ether compounds (JP-A-02-153903); alkylene oxide adducts of monohydric alcohols, or organic acid salts, or lactams (EP 555692); and phosphoric acid (Publication of Internal Patent Application as entered the national phase in Japan (Kohyo) No. 08-508517).
However, there are cases where conventional arts of crosslinking the neighborhood of the surface of the water-absorbent resin cannot sufficiently meet the abilities as demanded to the water-absorbent resin to a high degree in recent years. For example, in recent years, the sanitary materials have a tendency to be designed to be thinned, so the water-absorbent resin concentration in the absorbent structure becomes high. Thus, excellent absorption capacities under no load and under a heavy load with good balance are exemplified as the properties that are desired to the water-absorbent resin in the absorbent structure which contains the water-absorbent resin in a large quantity, namely, high concentration, but the above conventional methods are still insufficient to further raise each of the above values.
Furthermore, there is a problem of the safety of the crosslinking agent as used. Generally, as the molecular weights of compounds decrease, their perviousness from skin increases. In addition, when the crosslinking agent has high reactive groups such as epoxy group, the crosslinking agent itself has property to stimulate skin, so it is preferable to use a crosslinking agent of high molecular weight in view of the safety. However, there have been problems so far in that the use of a high molecular crosslinking agent with an epoxy group makes it very difficult to improve the absorption properties under a load. In other words, it is also difficult to achieve the safety and the absorption properties simultaneously. Then this problem is great especially when the crosslinking agent with an epoxy group.
In addition, there are also known methods in which the following other high molecular crosslinking agents are, for example, used: those which have both epoxy and amino groups (JP-A-61-293228), cationic ones (U.S. Pat. No. 5,382,610), polyamidoepichlorohydrin adducts or polyamineepichlorohydrin adducts (U.S. Pat. No. 5,597,872, JP-A-06-287220). However, these high molecular crosslinking agents all have a cationic group to form complexes with water-absorbent resins having an anionic group, so the uniform crosslinking is difficult to carry out, and the absorption capacity under a heavy load might be difficult to enhance. In addition, the high molecules as used in U.S. Pat. No. 5,597,872, JP-A-06-287220 are also still insufficient to the safety, because low molecular epichlorohydrin to be added tends to remain in the high molecules and therefore increases the halogen content of the water-absorbent resin by being added thereto.
Therefore, as things are, the above prior arts still do not satisfy either the aspect of safety or the properties of the resin.